Time to Contact: The optical Variable Tau
In situations in which a person moves toward an object to make contact with it, or the object moves toward the person, such as when a person is catching or hitting a thrown ball, vision plays an important role in specifying when to initiate the action and make contact with the object. The important visual information in these situations is the time to contact, which is the amount of time remaining until the object contact is the person (or vice versa) from specific distance (see Bootsma & Peper, 1992). Time to contact is specified according to the relative rate of change of the size of the image of the object on the retina of the eye. As the person approaches the object, or vice versa, the object produces an increasingly larger retina image. When this image attains a certain critical size, it triggers the action required by the situation.